chore(deps): ignore chokidar in dependabot (#21564)

## Context

Dependabot keeps proposing a chokidar v3 to v4 bump (e.g. #21555), but
chokidar is intentionally pinned to v3 via a `resolutions` entry in the
root `package.json`.

chokidar v4 removed the native macOS FSEvents backend and falls back to
`node:fs.watch`, which hits `EMFILE` on a repo this size. That was
diagnosed and fixed in #20316. The bump can't even be satisfied while
the resolution is in place, so the lockfile and `package.json` end up
inconsistent and CI goes red.

## Change

Add `chokidar` to the dependabot ignore list so it stops re-proposing
the bump every cycle. The pin stays until upstream restores a native
watcher backend.

Closes out #21555.

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.github/dependabot.yml

@@ -15,3 +15,6 @@ updates:
       - dependency-name: "@nestjs/graphql"
       - dependency-name: "@ptc-org/nestjs-query-graphql"
       - dependency-name: "typeorm"
+      # chokidar is pinned to v3 via a resolution: v4+ dropped native macOS
+      # FSEvents and hits EMFILE on a repo this size (see #20316)
+      - dependency-name: "chokidar"